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The dependence of the etching property of CoSi2 films in diluted HF solutions on the formation conditions
Applied Surface Science 178 (2001) 44±49
The dependence of the etching property of CoSi2 ®lms in diluted HF solutions on the formation conditions Shiyang Zhua,*, Guoping Rua, C. Detavernierb, R.L. Van Meirhaegheb, F. Cardonb, Bingzong Lia b
a Department of Electronic Engineering, Fudan University, Shanghai 200433, China Department of Solid State Sciences, Ghent University, Krijgslaan 281/S1, Gent B-9000, Belgium
Received 5 January 2001; accepted 15 March 2001
Abstract Three series of CoSi2 thin ®lms which were fabricated by solid state reaction of evaporated Co, Ti/Co and Co/Ti on p-type Si (1 0 0) substrates with different annealing procedures, have been etched in a diluted hydrogen ¯uoride (0.5 wt.% HF) solution at 308C. The etching rate ranges from 0.08 to 0.36 nm/s and the induction period (a period in the beginning of the etching process when there is no change in the sheet resistance value) ranges from 48 to 660 s for all those samples. The CoSi2 ®lms formed with a Ti capping layer depends on the annealing parameters more signi®cantly than other two series. The induction period reduces dramatically, but does not eliminate, if a previous step of selective etching in the H2O2±H2SO4 solution and (or) the NH3±H2O2±H2O solution is performed. The surface roughness of the etched samples is monitored by atomic force microscopy (AFM) measurements. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CoSi2 ®lms; Etching; Atomic force microscopy
1. Introduction CoSi2 is an important candidate for applications as contacts and interconnects in microelectronics due to its low resistivity, high thermal stability and absence of nucleation problems in narrow lines [1]. The common method to fabricate CoSi2 is solid state reaction of a deposited Co single layer or a Co, Ti bilayer with a Si substrate. It is well known that the reaction of the Co single layer with the Si substrate results in a polycrystalline CoSi2. While an epitaxial CoSi2 layer
*
Corresponding author. Tel.: 86-21-65643764/extn. 2; fax: 86-21-65648267. E-mail address: [email protected] (S. Zhu).
can be formed by adding a thin Ti interlayer and using a suitable annealing procedure (titanium interlayer mediated epitaxy, TIME) [2,3]. A Ti capping layer on Co causes a polycrystalline CoSi2, but with strong preferential orientation to that of the Si substrate [3]. The interaction of a silicide ®lm with chemicals and reactive gases during further processing is an important issue to maintain the quality of the integrated structure. CoSi2 is known to be robust with chemicals such as NH3- and H2SO4-based solutions. For this reason, H2 SO4 H2 O2 mixtures or NH3 H2 O2 H2 O mixtures are usually used for selective removal of the unreacted Co during silicidation process. However, a HF-based solution is an effective etchant for CoSi2. Maex et al. reported the kinetics and mechanism of CoSi2 etching in HF-based solutions [4,5].
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 4 5 - 8
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
They found that the etching rate of CoSi2 depends on the properties of the solution signi®cantly, such as the concentration of the adsorbed H and [HF]i, the temperature, the pH value, etc. while the CoSi2 ®lms were kept the same in their experiments. However, we found that the etching properties of CoSi2 also depend on its formation parameters. The etching properties of various CoSi2 in the same solution are compared in this paper. 2. Experimental details CoSi2 ®lms were formed on p-type (1 0 0)-oriented silicon. Immediately after a standard RCA cleaning followed by a diluted HF dip for removal of the native oxide layer, the wafers were loaded in a deposition system. Three series of samples, i.e. 20 nm Co, 20 nm Co 20 nm Ti (capping layer), 10 nm Ti (interlayer layer 20 nm Co, were deposited by e-gun evaporation sequentially without breaking the vacuum. The base pressure was better than 5 10 4 Pa. The silicidation was carried out in an AST rapid thermal annealing processor in a nitrogen ambient using one of the following procedures: (1) a single step annealing, (2) a single step annealing followed by a selective etch (SE), (3) two step annealing with a SE in between. The SE was carried out in a boiling H2SO4:H2 O2 4:1 solution for 10 s (SE1) and (or) in a boiling NH3:H2O2:H2 O 1:1:5 solution for 10 s (SE2). The silicon wafers for etching experiments were divided in rectangular samples with surface area of 5±9 cm2. A diluted HF solution with concentration of 0.5% (1 part commercial HF 40% and 200 part DIwater) or 2% HF was used and the temperature of the solution kept at 308C. The thinning of the CoSi2 layers was monitored by four-point probe sheet resistance measurements, i.e. the samples were etched and measured alternately until the sheet resistance value close to that of the bare silicon. For comparison, the thinning of the CoSi2 was also measured by a a-step contour graph. In those experiments, part of the sample surface was covered by a photoresist. After etching, the photoresist was removed and the step was measured. The surface roughness of the etched samples was characterized by atomic force microscopy (AFM).
45
3. Results and discussion Fig. 1 shows the etched thickness of the CoSi2 ®lm as a function of etching time in a HF 0.5% solution at 308C. The samples were formed from a Ti(20 nm)/ Co(20 nm)/Si system with one of the following annealing procedures, sample 1: RTA at 8508C for 30 s SE1; sample 2: RTA1 SE1 RTA2 at 8508C for 30 s; and sample 3: RTA1 RTA2 at 10508C for 30 s. The etched thickness (DT) in Fig. 1(a) is calculated from the sheet resistance (Rsq) using " # 1 1 DT r Rsq ini Rsq where r (0.16 O mm) is the resistivity of the CoSi2 and (Rsq)ini is the initial sheet resistance. For
Fig. 1. The etched thickness of three CoSi2 samples as a function of etching time in a HF 0.5% solution (a) sheet resistance results; (b) contour graph results.
46
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
comparison, Fig. 1(b) gives the etched thickness directly measured by a a-step contour graph, in agreement with the data of Fig. 1(a) roughly. We ®nd it is dif®cult to get exact value of the thickness variation using the contour graph measurement if the thickness drop is small, which has relatively large experimental error, as shown in Fig. 1(b). It seems the sheet resistance can get more reasonable results. Following, the results from the sheet resistance measurements are given. In Fig. 1, the etch rate of sample 1, 2 and 3 is 0.18, 0.17 and 0.08 nm/s, respectively. The difference in the etching rate between sample 3 and the other two samples is obvious. Moreover, there is a period in the beginning of the etching process when there is no change in the sheet resistance values. This period has been called as the ``induction period'' and has been regarded to be related to an as-grown surface layer, which is formed during silicidation. The induction period is 37, 133 and 240 s for those three samples respectively, depending on the silicidation procedure signi®cantly. Increasing the HF concentration of the solution increases the etching rate and decreases the induction period simultaneously. Fig. 2 shows the etched thickness as a function of etching time of sample 2 and 3 in a 2% HF solution at 308C. The etching rate increases to 0.52 and 0.21 nm/s for sample 2 and 3 and the corresponding induction period decreases to 27 and 77 s, respectively. However, the relationship of different samples does not change in the different solutions, i.e. the etching rate is always smaller and the induction period is always longer for sample 3 than that for
Fig. 2. The etched thickness of two CoSi2 samples as a function of etching time in a HF 2% solution.
sample 2. For simplicity, we compare the various CoSi2 in the same solution at the same temperature. Fig. 3 shows the etching properties of three series CoSi2 samples which are formed from Co(20 nm)/Si,
Fig. 3. The etched thickness of three series samples as a function of etching time in a HF 0.5% solution for different silicidation parameters.
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
Ti(10 nm)/Co(20 nm)/Si and Co(20 nm)/Ti(10 nm)/Si respectively, identi®ed as R-, C- and E-series, respectively. The silicidation procedures for R- and C-series are: (1) RTA at 8508C for 30 s; (2) RTA at 10508C for 30 s; (3) RTA at 4758C for 30 s SE2 RTA at 8508C for 30 s; (4) RTA at 4758C for 30 s SE1 SE2 RTA at 8508C for 30 s; (5) RTA at 4758C for 30 s SE2 RTA at 10508C for 30 s; (6) RTA at 4758C for 30 s SE1 SE2 RTA at 10508C for 30 s. In order to form high quality epitaxial CoSi2, the annealing procedures of E-series are: (1) RTA at 8508C for 30 s; (2) RTA at 850 C RTA at 11008C for 30 s; (3) RTA at 8508C for 30 s SE2; (4) RTA at 8508C for 30 s SE2 RTA at 11008C for 30 s; (5) RTA at 8508C for 30 s SE1 SE2; (6) RTA at 8508C for 30 s SE1 SE2 RTA at 11008C for 30 s. For polycrystalline CoSi2 formed from Co±Si reaction (Fig. 3(a)), the etching rate is about 0.23±0.26 nm/s, independent of the annealing procedure. The difference in the induction period is slightly large, about 300 s for samples R1 and R3 and about 200 s for the other four samples. It may be due to the fact that the increase of the annealing temperature reduces the as-grown surface layer. This upper layer cannot be etched by the NH3-based solution, but can be slightly reduced by the H2SO4-based solution. Signi®cant inhomogeneity in the sample surface is observed during etching for all those samples. We measured three locations in the same sample to average the sheet resistance value. But the data still have large deviation, as shown in Fig. 3(a). This may be attributed to the non-uniformity of the upper layer thickness. Since the removal of the upper layer is much slower than the etching of the CoSi2, the slight difference in the upper layer thickness will cause signi®cant difference in the etching process of the sub-layer CoSi2. On the other hand, signi®cant differences in both the etching rate and the induction period are observed for the C-series samples. The induction period is about 650 s for samples C1 and C2 and about 220±300 s for the other four samples. The long induction period for C1 and C2 can be attributed to the TiN or other Tirelated layer on the surface in the condition of Ti/Co/ Si reaction. It is known that this layer can be etched by the H2SO4-based solution and (or) the NH3-based solution. After this layer is removed, the etching properties are similar to those of R-series samples.
47
Signi®cant inhomogeneity is also observed for Cseries samples during the etching in the diluted HF solution. The etching rate of sample C2 is about 0.1 nm/s, lower than the other samples (about 0.17± 0.24 nm/s), in agreement with the result of Fig. 1. The etching properties of the epitaxial CoSi2 are shown in Fig. 3(c). Unlike the polycrystalline CoSi2, epitaxial CoSi2 layer keeps uniform during etching in the HF solution. Moreover, we ®nd that the dependence of the inverse value of the sheet resistance on the etching time is somewhat parabolic rather than linear. The induction period can be reduced by the selective etching in the NH3-based solution and further reduced by additional selective etching in the H2SO4 solution. The initial CoSi2 thickness increases after the second RTA at 11008C. It is known that the CoSi2 formation temperature is higher in the Co/Ti/Si system than in the Co/Si system [3]. During the additional high temperature annealing, the Co±Ti± Si reaction continues and thick CoSi2 can be formed. As expected, the increase of the CoSi2 thickness is reduced after selective etching. In Fig. 3(c) the approximate etching rate for epitaxial CoSi2 is about 0.28±0.30 nm/s at the latter part of the etching. The slightly larger etching rate, as well as the non-linearity of the etching rate may be not only from the thickness variation itself but also from the CoSi2 resistivity variation. It is known that there is Ti incorporation in the epitaxial CoSi2 ®lm near the CoSi2/Si interface and the CoxTiySi2 ternary has higher resistivity than
Fig. 4. The etched thickness of epitaxial CoSi2 samples with or without selective etching after silicidation as a function of etching time.
48
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
that of CoSi2 [3]. Therefore, the etched thickness in Fig. 3(c) may be overestimated with increasing etching time. Similar to the C-series samples, the induced period is also reduced after the selective etching. Fig. 4
shows it more clearly. The silicidation was carried out by one step annealing at 10508C for complete reaction. The selective etching after silicidation decreases the induction period from about 220 to about 40 s.
Fig. 5. AFM topographies of a CoSi2 sample surface (a) before and after etching in a HF 0.5% solution for (b) 240 s; (c) 480 s; (d) 600 s; (e) 720 s and (f) 840 s.
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
The surface roughness of the etched sample was measured by AFM. Within a small size (5 mm 5 mm) the surface roughness only increases slightly after etching in the diluted HF solution, even the large inhomogeneity of the sample surface can be observed by the bare eye and (or) be measured by the four point probe sheet resistance. Fig. 5(a)±(f) show the AFM topographies of a sample before and after etching in the diluted HF solution for 240, 480, 600, 720 and 840 s, respectively. The sample was formed by solid state reaction of (20 nm)Co/Si at 8508C with a selective etching. The surface roughness depends on the etching time weakly. The other samples also have similar behavior. It implies that the inhomogeneity of etching may come from the non-uniformity of the upper layer, rather than the CoSi2 ®lm itself. Moreover, we ®nd that the delineation of the grain boundaries of the silicide ®lm becomes clearer with increasing the etching time. After 840 s etching, the CoSi2 ®lm is completely removed and the footprints of the silicide grains on the silicon substrate surface are observed. The result implies that the etching mechanism of the CoSi2 ®lm is layer by layer with a little preferential etching at the grain boundaries, in agreement with the conclusion obtained by Donaton et al. [4]. 4. Summary The etching of various CoSi2 ®lms in a diluted HF solution was studied. The dependence of the etching
49
properties (the etching rate, the induction period and the etching uniformity) on the CoSi2 ®lm fabrication parameters are observed. The induction period for the samples formed from solid state reaction of a Ti, Co bilayer and Si can be reduced dramatically, but cannot be eliminated completely, by the selective etching during or after silicidation. The etching rate is also in¯uenced by the annealing procedure. Polycrystalline CoSi2 ®lms show signi®cant inhomogeneity during etching, while the etching of the epitaxial CoSi2 ®lm is quite homogenous. Acknowledgements The work is supported by National Natural Science Foundation of China under grant no. 69876007 and the Science and Technology Development Foundation of Shanghai under grant no. 98JC14004. References [1] K. Maex, Mater. Sci. Eng. R 11 (2/3) (1993). [2] P. Liu, B.Z. Li, Z. Shen, Z.G. Gu, W.N. Wang, Z.Y. Zhou, R.S. Ni, C.L. Lin, S.C. Zou, F. Hong, G.A. Rozgonyi, J. Appl. Phys. 74 (3) (1993) 1700. [3] C. Detavernier, R.L. Van Meirhaeghe, F. Cardon, K. Maex, H. Bender, S.Y. Zhu, J. Appl. Phys. 88 (1) (2000) 133. [4] R.A. Donaton, K. Lokere, R. Verbeeck, K. Maex, Appl. Surf. Sci. 89 (1995) 221. [5] M.R. Baklanov, I.A. Badmaeva, R.A. Donaton, L.L. Sveshnikova, W. Storn, K. Maex, J. Electrochem. Soc. 143 (10) (1996) 3245.
The dependence of the etching property of CoSi2 ®lms in diluted HF solutions on the formation conditions Shiyang Zhua,*, Guoping Rua, C. Detavernierb, R.L. Van Meirhaegheb, F. Cardonb, Bingzong Lia b
a Department of Electronic Engineering, Fudan University, Shanghai 200433, China Department of Solid State Sciences, Ghent University, Krijgslaan 281/S1, Gent B-9000, Belgium
Received 5 January 2001; accepted 15 March 2001
Abstract Three series of CoSi2 thin ®lms which were fabricated by solid state reaction of evaporated Co, Ti/Co and Co/Ti on p-type Si (1 0 0) substrates with different annealing procedures, have been etched in a diluted hydrogen ¯uoride (0.5 wt.% HF) solution at 308C. The etching rate ranges from 0.08 to 0.36 nm/s and the induction period (a period in the beginning of the etching process when there is no change in the sheet resistance value) ranges from 48 to 660 s for all those samples. The CoSi2 ®lms formed with a Ti capping layer depends on the annealing parameters more signi®cantly than other two series. The induction period reduces dramatically, but does not eliminate, if a previous step of selective etching in the H2O2±H2SO4 solution and (or) the NH3±H2O2±H2O solution is performed. The surface roughness of the etched samples is monitored by atomic force microscopy (AFM) measurements. # 2001 Elsevier Science B.V. All rights reserved. Keywords: CoSi2 ®lms; Etching; Atomic force microscopy
1. Introduction CoSi2 is an important candidate for applications as contacts and interconnects in microelectronics due to its low resistivity, high thermal stability and absence of nucleation problems in narrow lines [1]. The common method to fabricate CoSi2 is solid state reaction of a deposited Co single layer or a Co, Ti bilayer with a Si substrate. It is well known that the reaction of the Co single layer with the Si substrate results in a polycrystalline CoSi2. While an epitaxial CoSi2 layer
*
Corresponding author. Tel.: 86-21-65643764/extn. 2; fax: 86-21-65648267. E-mail address: [email protected] (S. Zhu).
can be formed by adding a thin Ti interlayer and using a suitable annealing procedure (titanium interlayer mediated epitaxy, TIME) [2,3]. A Ti capping layer on Co causes a polycrystalline CoSi2, but with strong preferential orientation to that of the Si substrate [3]. The interaction of a silicide ®lm with chemicals and reactive gases during further processing is an important issue to maintain the quality of the integrated structure. CoSi2 is known to be robust with chemicals such as NH3- and H2SO4-based solutions. For this reason, H2 SO4 H2 O2 mixtures or NH3 H2 O2 H2 O mixtures are usually used for selective removal of the unreacted Co during silicidation process. However, a HF-based solution is an effective etchant for CoSi2. Maex et al. reported the kinetics and mechanism of CoSi2 etching in HF-based solutions [4,5].
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 4 5 - 8
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
They found that the etching rate of CoSi2 depends on the properties of the solution signi®cantly, such as the concentration of the adsorbed H and [HF]i, the temperature, the pH value, etc. while the CoSi2 ®lms were kept the same in their experiments. However, we found that the etching properties of CoSi2 also depend on its formation parameters. The etching properties of various CoSi2 in the same solution are compared in this paper. 2. Experimental details CoSi2 ®lms were formed on p-type (1 0 0)-oriented silicon. Immediately after a standard RCA cleaning followed by a diluted HF dip for removal of the native oxide layer, the wafers were loaded in a deposition system. Three series of samples, i.e. 20 nm Co, 20 nm Co 20 nm Ti (capping layer), 10 nm Ti (interlayer layer 20 nm Co, were deposited by e-gun evaporation sequentially without breaking the vacuum. The base pressure was better than 5 10 4 Pa. The silicidation was carried out in an AST rapid thermal annealing processor in a nitrogen ambient using one of the following procedures: (1) a single step annealing, (2) a single step annealing followed by a selective etch (SE), (3) two step annealing with a SE in between. The SE was carried out in a boiling H2SO4:H2 O2 4:1 solution for 10 s (SE1) and (or) in a boiling NH3:H2O2:H2 O 1:1:5 solution for 10 s (SE2). The silicon wafers for etching experiments were divided in rectangular samples with surface area of 5±9 cm2. A diluted HF solution with concentration of 0.5% (1 part commercial HF 40% and 200 part DIwater) or 2% HF was used and the temperature of the solution kept at 308C. The thinning of the CoSi2 layers was monitored by four-point probe sheet resistance measurements, i.e. the samples were etched and measured alternately until the sheet resistance value close to that of the bare silicon. For comparison, the thinning of the CoSi2 was also measured by a a-step contour graph. In those experiments, part of the sample surface was covered by a photoresist. After etching, the photoresist was removed and the step was measured. The surface roughness of the etched samples was characterized by atomic force microscopy (AFM).
45
3. Results and discussion Fig. 1 shows the etched thickness of the CoSi2 ®lm as a function of etching time in a HF 0.5% solution at 308C. The samples were formed from a Ti(20 nm)/ Co(20 nm)/Si system with one of the following annealing procedures, sample 1: RTA at 8508C for 30 s SE1; sample 2: RTA1 SE1 RTA2 at 8508C for 30 s; and sample 3: RTA1 RTA2 at 10508C for 30 s. The etched thickness (DT) in Fig. 1(a) is calculated from the sheet resistance (Rsq) using " # 1 1 DT r Rsq ini Rsq where r (0.16 O mm) is the resistivity of the CoSi2 and (Rsq)ini is the initial sheet resistance. For
Fig. 1. The etched thickness of three CoSi2 samples as a function of etching time in a HF 0.5% solution (a) sheet resistance results; (b) contour graph results.
46
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
comparison, Fig. 1(b) gives the etched thickness directly measured by a a-step contour graph, in agreement with the data of Fig. 1(a) roughly. We ®nd it is dif®cult to get exact value of the thickness variation using the contour graph measurement if the thickness drop is small, which has relatively large experimental error, as shown in Fig. 1(b). It seems the sheet resistance can get more reasonable results. Following, the results from the sheet resistance measurements are given. In Fig. 1, the etch rate of sample 1, 2 and 3 is 0.18, 0.17 and 0.08 nm/s, respectively. The difference in the etching rate between sample 3 and the other two samples is obvious. Moreover, there is a period in the beginning of the etching process when there is no change in the sheet resistance values. This period has been called as the ``induction period'' and has been regarded to be related to an as-grown surface layer, which is formed during silicidation. The induction period is 37, 133 and 240 s for those three samples respectively, depending on the silicidation procedure signi®cantly. Increasing the HF concentration of the solution increases the etching rate and decreases the induction period simultaneously. Fig. 2 shows the etched thickness as a function of etching time of sample 2 and 3 in a 2% HF solution at 308C. The etching rate increases to 0.52 and 0.21 nm/s for sample 2 and 3 and the corresponding induction period decreases to 27 and 77 s, respectively. However, the relationship of different samples does not change in the different solutions, i.e. the etching rate is always smaller and the induction period is always longer for sample 3 than that for
Fig. 2. The etched thickness of two CoSi2 samples as a function of etching time in a HF 2% solution.
sample 2. For simplicity, we compare the various CoSi2 in the same solution at the same temperature. Fig. 3 shows the etching properties of three series CoSi2 samples which are formed from Co(20 nm)/Si,
Fig. 3. The etched thickness of three series samples as a function of etching time in a HF 0.5% solution for different silicidation parameters.
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
Ti(10 nm)/Co(20 nm)/Si and Co(20 nm)/Ti(10 nm)/Si respectively, identi®ed as R-, C- and E-series, respectively. The silicidation procedures for R- and C-series are: (1) RTA at 8508C for 30 s; (2) RTA at 10508C for 30 s; (3) RTA at 4758C for 30 s SE2 RTA at 8508C for 30 s; (4) RTA at 4758C for 30 s SE1 SE2 RTA at 8508C for 30 s; (5) RTA at 4758C for 30 s SE2 RTA at 10508C for 30 s; (6) RTA at 4758C for 30 s SE1 SE2 RTA at 10508C for 30 s. In order to form high quality epitaxial CoSi2, the annealing procedures of E-series are: (1) RTA at 8508C for 30 s; (2) RTA at 850 C RTA at 11008C for 30 s; (3) RTA at 8508C for 30 s SE2; (4) RTA at 8508C for 30 s SE2 RTA at 11008C for 30 s; (5) RTA at 8508C for 30 s SE1 SE2; (6) RTA at 8508C for 30 s SE1 SE2 RTA at 11008C for 30 s. For polycrystalline CoSi2 formed from Co±Si reaction (Fig. 3(a)), the etching rate is about 0.23±0.26 nm/s, independent of the annealing procedure. The difference in the induction period is slightly large, about 300 s for samples R1 and R3 and about 200 s for the other four samples. It may be due to the fact that the increase of the annealing temperature reduces the as-grown surface layer. This upper layer cannot be etched by the NH3-based solution, but can be slightly reduced by the H2SO4-based solution. Signi®cant inhomogeneity in the sample surface is observed during etching for all those samples. We measured three locations in the same sample to average the sheet resistance value. But the data still have large deviation, as shown in Fig. 3(a). This may be attributed to the non-uniformity of the upper layer thickness. Since the removal of the upper layer is much slower than the etching of the CoSi2, the slight difference in the upper layer thickness will cause signi®cant difference in the etching process of the sub-layer CoSi2. On the other hand, signi®cant differences in both the etching rate and the induction period are observed for the C-series samples. The induction period is about 650 s for samples C1 and C2 and about 220±300 s for the other four samples. The long induction period for C1 and C2 can be attributed to the TiN or other Tirelated layer on the surface in the condition of Ti/Co/ Si reaction. It is known that this layer can be etched by the H2SO4-based solution and (or) the NH3-based solution. After this layer is removed, the etching properties are similar to those of R-series samples.
47
Signi®cant inhomogeneity is also observed for Cseries samples during the etching in the diluted HF solution. The etching rate of sample C2 is about 0.1 nm/s, lower than the other samples (about 0.17± 0.24 nm/s), in agreement with the result of Fig. 1. The etching properties of the epitaxial CoSi2 are shown in Fig. 3(c). Unlike the polycrystalline CoSi2, epitaxial CoSi2 layer keeps uniform during etching in the HF solution. Moreover, we ®nd that the dependence of the inverse value of the sheet resistance on the etching time is somewhat parabolic rather than linear. The induction period can be reduced by the selective etching in the NH3-based solution and further reduced by additional selective etching in the H2SO4 solution. The initial CoSi2 thickness increases after the second RTA at 11008C. It is known that the CoSi2 formation temperature is higher in the Co/Ti/Si system than in the Co/Si system [3]. During the additional high temperature annealing, the Co±Ti± Si reaction continues and thick CoSi2 can be formed. As expected, the increase of the CoSi2 thickness is reduced after selective etching. In Fig. 3(c) the approximate etching rate for epitaxial CoSi2 is about 0.28±0.30 nm/s at the latter part of the etching. The slightly larger etching rate, as well as the non-linearity of the etching rate may be not only from the thickness variation itself but also from the CoSi2 resistivity variation. It is known that there is Ti incorporation in the epitaxial CoSi2 ®lm near the CoSi2/Si interface and the CoxTiySi2 ternary has higher resistivity than
Fig. 4. The etched thickness of epitaxial CoSi2 samples with or without selective etching after silicidation as a function of etching time.
48
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
that of CoSi2 [3]. Therefore, the etched thickness in Fig. 3(c) may be overestimated with increasing etching time. Similar to the C-series samples, the induced period is also reduced after the selective etching. Fig. 4
shows it more clearly. The silicidation was carried out by one step annealing at 10508C for complete reaction. The selective etching after silicidation decreases the induction period from about 220 to about 40 s.
Fig. 5. AFM topographies of a CoSi2 sample surface (a) before and after etching in a HF 0.5% solution for (b) 240 s; (c) 480 s; (d) 600 s; (e) 720 s and (f) 840 s.
S. Zhu et al. / Applied Surface Science 178 (2001) 44±49
The surface roughness of the etched sample was measured by AFM. Within a small size (5 mm 5 mm) the surface roughness only increases slightly after etching in the diluted HF solution, even the large inhomogeneity of the sample surface can be observed by the bare eye and (or) be measured by the four point probe sheet resistance. Fig. 5(a)±(f) show the AFM topographies of a sample before and after etching in the diluted HF solution for 240, 480, 600, 720 and 840 s, respectively. The sample was formed by solid state reaction of (20 nm)Co/Si at 8508C with a selective etching. The surface roughness depends on the etching time weakly. The other samples also have similar behavior. It implies that the inhomogeneity of etching may come from the non-uniformity of the upper layer, rather than the CoSi2 ®lm itself. Moreover, we ®nd that the delineation of the grain boundaries of the silicide ®lm becomes clearer with increasing the etching time. After 840 s etching, the CoSi2 ®lm is completely removed and the footprints of the silicide grains on the silicon substrate surface are observed. The result implies that the etching mechanism of the CoSi2 ®lm is layer by layer with a little preferential etching at the grain boundaries, in agreement with the conclusion obtained by Donaton et al. [4]. 4. Summary The etching of various CoSi2 ®lms in a diluted HF solution was studied. The dependence of the etching
49
properties (the etching rate, the induction period and the etching uniformity) on the CoSi2 ®lm fabrication parameters are observed. The induction period for the samples formed from solid state reaction of a Ti, Co bilayer and Si can be reduced dramatically, but cannot be eliminated completely, by the selective etching during or after silicidation. The etching rate is also in¯uenced by the annealing procedure. Polycrystalline CoSi2 ®lms show signi®cant inhomogeneity during etching, while the etching of the epitaxial CoSi2 ®lm is quite homogenous. Acknowledgements The work is supported by National Natural Science Foundation of China under grant no. 69876007 and the Science and Technology Development Foundation of Shanghai under grant no. 98JC14004. References [1] K. Maex, Mater. Sci. Eng. R 11 (2/3) (1993). [2] P. Liu, B.Z. Li, Z. Shen, Z.G. Gu, W.N. Wang, Z.Y. Zhou, R.S. Ni, C.L. Lin, S.C. Zou, F. Hong, G.A. Rozgonyi, J. Appl. Phys. 74 (3) (1993) 1700. [3] C. Detavernier, R.L. Van Meirhaeghe, F. Cardon, K. Maex, H. Bender, S.Y. Zhu, J. Appl. Phys. 88 (1) (2000) 133. [4] R.A. Donaton, K. Lokere, R. Verbeeck, K. Maex, Appl. Surf. Sci. 89 (1995) 221. [5] M.R. Baklanov, I.A. Badmaeva, R.A. Donaton, L.L. Sveshnikova, W. Storn, K. Maex, J. Electrochem. Soc. 143 (10) (1996) 3245.